Under a real-time pricing rate structure, the focus of operating a chilled-water storage system is to determine the optimal starting and ending time of charging and discharging the tank. This paper proposes a simple method to determine the optimal operation. The chiller plant performance is simulated with a forward model based on wire-to-water efficiency to reflect the performance change with different chiller part-load ratios. The tank state is described with chilled-water volume, and the tank inventory change is described with tank chilledwater flow rate to ensure consistence in the whole cycle. The electricity price is simulated with an existing prediction model. The loop chilled-water delta-T is modeled with a linear regression model. This method is evaluated with an illustrative chilled-water storage system. Simulation results show that, compared to the operation costs without storage, the annual electric billing costs with the optimal tank operation can be reduced by almost one-third, the majority of which is achieved in summer. A near-optimal operation strategy is proposed by implementing the constant starting and ending time of charging and discharging cycles. The statistic results show that the annual billing cost savings only decrease by 2.2%.